Me and my wife have both got the NOKIA 6. We both have experienced low volume output via headphones. To make my self clear, in most cases the volume output is ok. But when being outside in very noisy environments, the audio volume output is just not enough at all. I have tried different headphones and had the same result with all of them.
Also I have compared NOKIA 6 against my old Lumia 640, Huawei P9 and iPhone 7 and it is behind all three in terms of volume level via headphones. It makes it impossible to answer a call and have a discussion (or listen to a song) when I am in the metro station where there is a lot of noise. I didn't have this issue with my old Lumia 640....
Please fix this case. It should not be a big issue, just a matter of some code tweak.
Check under audio setting see if someone mutted your windows sound. also check under sound devices the audio chipset and not gpu chipset set as main audio output. if the output still low try installing the newest audio driver see if the audio driver got damaged. also check to see if window or your media player audio set right.